Four patients with Ebola escaped after a Doctors without Borders (MSF) clinic in eastern Congo was attacked by unknown assailants, the Ministry of Health said on Thursday.

“Four patients confirmed to have the deadly virus who fled during the attack are still missing,’’ the ministry said, as well as others who were suspected of being infected.

The ministry said that the attack was the second on a health clinic within a few days, as aid agencies and government battle to contain the virus in an area rife with militia groups and public distrust.

So far 553 people have died of Ebola in the outbreak in eastern Congo, which began shortly after the country’s government in June declared an end to another outbreak in the west of the country.

Authorities and aid groups have been struggling to do their jobs in the dangerous eastern region where numerous militias operate, most fighting over the Central African country’s rich natural resources.

According to the health ministry, over 84,000 people have been vaccinated against the virus since August.

Ebola is a highly infectious disease that causes a fever and often leads to massive internal bleeding and death.